1. Field of the Invention
This invention relates to a fuel heating apparatus used in an engine adapted to ignite and burn a fuel (which will hereinafter be referred to by a generic name "heavy oil") of a high viscosity, such as a heavy oil injected into a combustion chamber by an operation of a fuel injection pump, and lower the viscosity of such a fuel by heating the same.
2. Description of the Prior Art
In general, light oil is burnt as a fuel in a fuel chamber in a diesel engine. The light oil has a high cetane value and not so high a viscosity, and is suitable to be uniformly distributed as an atomized fuel into a combustion chamber. In order to reduce the amounts of particulates and NOx component in an exhaust gas from a recent diesel engine to a very low level, the atomization of a fuel in a combustion chamber is attained by compressing the air under a high pressure in the combustion chamber, and injecting the fuel thereinto under a superhigh pressure. The fuels generally used for an engine include gasoline, light oil and heavy oil. The viscosity of gasoline is 0.1 cSt, light oil 1.8-2.7 cSt, A-heavy oil 20 cSt, B-heavy oil 50 cSt, and C-heavy oil 50-400 cSt. The cetane value of light oil is not lower than 40, while the cetane value of heavy oil is as low as around 25. The fuel cost of light oil is about 70, and that of heavy oil about 15 with respect to a base fuel cost 100 of, for example, gasoline.
Many of the conventional generating apparatuses comprising cogeneration systems are systems formed by mounting a generator on an engine. In the generation of electric power by a diesel engine, the fuel cost becomes high because a diesel engine uses light oil as a fuel, so that the advantageousness cannot be secured due to the high cost.
In a low-speed marine engine, the revolution speed of which is not more than 200 rpm, the time for mixing a fuel and air with each other is long, enabling a gaseous mixture to be made with use of a heavy oil. However, in a high-speed diesel engine, the revolution speed of which is not less than 1000 rpm, the time for forming a gaseous mixture becomes short, so that using a heavy oil as a fuel is very difficult. In a generating apparatus using a diesel engine, a high-speed diesel engine is used. If a heavy oil of a low fluidity can be ignited and burnt as a fuel in a high-speed diesel engine, the fuel cost can be reduced, and the power cost can also be lowered greatly, this oil being thereby proved useful. However, when the viscosity of a fuel is high like that of heavy oil, the atomization and injection thereof using a fuel injection pump, which is adapted to compress a fuel under a high pressure in a high-speed diesel engine, becomes difficult. Especially, when the temperature of the atmospheric air is low, the fluidity of the heavy oil is low under these circumstances a fuel injection pump cannot be used or can a heavy oil as a fuel be injected into a combustion chamber of the high-speed diesel engine. Especially, in a generating apparatus provided with a large-scale fuel tank, the problem concerning the viscosity of the heavy oil becomes serious when the atmospheric temperature is low.
In view of the above, if a heavy oil can be utilized as a fuel in a load dispatching device using a diesel engine, the fuel cost can be greatly reduced. Accordingly, it is conceivable that the heavy oil be heated to give a fluidity thereto for the purpose of injecting the heavy oil under a high pressure into a combustion chamber.
The applicant of the present invention developed diesel engines using a heavy oil as a fuel, and filed, for example, Japanese Patent Laid-Open No. 217624/1997 and Japanese Patent Application Nos. 151910/1996 and 151911/1996. In such diesel engines, a heavy oil as a fuel can be atomized excellently and injected into a combustion chamber by giving a fluidity thereto. Therefore, it is desired that a fluidity be given to a heavy oil. An issue of finding out a method of heating a heavy oil, which is supplied from a fuel tank, at a low cost and with a high effectiveness with the possibility of improving the fluidity of the heavy oil by heating the same taken into consideration then arise.
However, in an engine using a heavy oil as a fuel, the viscosity of the oil becomes so high that the oil becomes like thick malt syrup in a season of a low temperature, such as winter. In a marine engine, the fluidity of a fuel is improved in the existing circumstances by heating the same by a burner. Utilizing a burner to heat a heavy oil causes a fuel to be consumed, and the operating cost to increase in consequence. When a cooled diesel engine is used as a cogeneration system, hydrocarbons account for a greater part of the components of an exhaust gas from the engine, and, moreover, the exhaust gas maintains a temperature of around 500.degree. C. Consequently, the temperature of the atmospheric air in city and urban areas increases due to the that of exhaust gas as the cogeneration system spreads.
If a heavy oil can be heated by using a heat exchanger utilizing an exhaust gas from a diesel engine, so as to reduce the viscosity of the heavy oil, increase the injection pressure of a fuel injection pump and attain the atomization of a gaseous mixture, the fuel cost can be greatly reduced, and the thermal energy of the exhaust gas can be absorbed by the heat exchanger. Therefore, the temperature of the exhaust gas decreases, and a temperature rise problem posed by the exhaust gas can be solved. Since the thermal energy of the exhaust gas is absorbed by the heat exchanger, it becomes necessary to solve the problem of developing an efficient exhaust gas recovering method and a method of preventing the heat exchanger from being oxidized and corroded due to the exhaust gas.